16: The Type System: Structs

WARNING This exercise is in DRAFT status, so there may be errors. If you find any, please email me at help@learncodethehardway.com so I can fix them.

From now on everything you'll learn about Go is some combination of--or is very similar to--something you already know. For example, Structs (the topic of this exercise) are nothing more than a collection of variables under one name. You alread know what a variable is, and how variables have a type (int, float32, etc.), so struct is nothing more than a grouping of those.

Another way to describe a struct is, "A new type that is a collection of other types." You can also think of a struct as a single row from a database or spreadsheet. Each column of the row has a name and a type (INTEGER, FLOAT, TEXT) and acts as a cohesive collection of those columns.

Using struct

Let's look at a small example:

type Position struct {
    X int
    Y int
}

Here we have a Position struct, and as you can see we create it with the type keyword. That identifies it as a new type similar to int or float32, but Position is composed of two fields (aka variables) X and Y.

This Position type can then be used as the type of a variable like so:

// declaring a variable as Position
var player_at Position

// or, with the := like so
enemy_at := Position{10,10}

To access the fields (contents) of the struct you use the . (dot) similar to how you use it with fmt.Println. The . (dot) usually means "get from" or "access" in many languages. For example, to get the position of an enemy I would do:

x_field := enemy_at.X
y_field := enemy_at.Y

Printing the coordinate would look something like this:

fmt.Println("x=", enemy_at.x, "y=", enemy_at.y)

Finally, you can set any field to a new value by simply treating it like a variable and use = (but not :=):

enemy_at.X = 100
enemy_at.Y = 20

The Code

Here's the code for this exercise, where we make some Animal structs and do things with the fields:

View Source file ex16/main.go Only

package main

import (
    "fmt"
)

type Animal struct {
    Name string
    Age int
    Species string
    Weight float64
}

func PrintAnimal(who Animal) {
    // you can pass your structs directly to Println
    fmt.Println("Your Pet:", who)
}

func main() {
    frankie := Animal{"Fankie", 5, "Dog", 20.5};

    // access a field in a struct with .
    fmt.Println("Name:", frankie.Name);

    PrintAnimal(frankie)

    // change a field by assigning to it
    frankie.Weight = 25.6
    fmt.Println("Your Pet Gained Weight:", frankie)

    // make a copy of a struct
    miss_snuffles := frankie

    // test if they're equal
    if(miss_snuffles == frankie) {
        // change it with the . and =
        miss_snuffles.Name = "Miss Snuffles"
        miss_snuffles.Species = "Cat"
        miss_snuffles.Weight = 4.0
    }

    // pass structs to functions
    PrintAnimal(frankie)
    PrintAnimal(miss_snuffles)
}

The Breakdown

As usual I'm only going to describe lines that are new to you.

• type Animal struct { -- Create the Animal struct.
• Name string -- Name field of type string
• Age int -- Age field of type int
• Species string -- Species field of type string
• Weight float64 -- Weight field of type float64
• } -- Ends the struct
• func PrintAnimal(who Animal) { -- Passing an animal to a function.
• frankie := Animal{"Fankie", 5, "Dog", 20.5}; -- Create an animal that starts off with Name="Frankie" and so on.
• fmt.Println("Name:", frankie.Name); -- Print the Name.
• PrintAnimal(frankie) -- Call PrintAnimal with frankie
• frankie.Weight = 25.6 -- Change frankie's Weight to 25.6
• fmt.Println("Your Pet Gained Weight:", frankie) -- Shame frankie for gaining weight.
• miss_snuffles := frankie -- Make a copy of frankie.
• if(miss_snuffles == frankie) { -- Test that two structs are equal.
• miss_snuffles.Name = "Miss Snuffles" -- Change this new Animal to have different settings.
• miss_snuffles.Species = "Cat" -- It's a cat.
• miss_snuffles.Weight = 4.0 -- Cats are small.
• PrintAnimal(frankie) -- Print frankie again.
• PrintAnimal(miss_snuffles) -- Print miss_snuffles, so you can see how it changed.

Make sure you understand each line, and if there any you don't then try to figure out why. Sometimes experimenting with lines of code you don't get will help you understand it.

The Practice

1. Break It -- Try assigning to fields with the wrong names, give them the wrong types (like try to set an int to a string).
2. Change It -- Can you add to Animal? Can you create another struct?
3. Remake It -- As usual, write a description the recreate it from memory using the description. The more you do this, the better you'll get.

Study Drills

From now on I'm going to point you at the Go Specification and ask you to read it and try to understand it. You may not understand everything there, but it's good to know where it is.

1. Read the Go Specification for struct.
2. Add a struct that is inside the Animal struct as a new field.
3. Try using fmt.Println directly on the frankie variable.
4. Using what you know about reading input from the last module, can you store a user's input into a struct?